There has been known a lane recognition device which recognizes a lane mark representing a traffic lane boundary provided on a mad surface using visual sensor information mounted in a vehicle and recognizes a relative positional relationship between an own vehicle and a lane.
As one of the lane marks representing the traffic lane boundary, there is a “linear lane mark” such as a continuous line and a dashed line painted in white, yellow, or the like in color on a mad surface.
In addition to the linear lane mark, there is a “stud-shaped (type) lane mark” in which reflective elements and so on are buried along a lane boundary. The stud-shaped lane mark is called a point sequential lane mark, a cat's eye, a botts dot, a road stud, and so on. The botts dot (also referred to as Bott' dot(s) or Bott dot(s)) is a kind of the stud-shaped lane mark used especially in California, United States of American. In the present specification and the present invention, the “stud-shaped lane mark” is a general term including the above exemplified lane marks, and the “stud shape” does not limit specific shapes.
As an example of such a lane mark recognition device, Patent Document 1 discloses a lane recognition apparatus especially for the linear lane mark.
The lane recognition apparatus disclosed in the Patent Document 1 includes an image input device, an edge emphasizing device, a straight line extraction processing device, a white line determination processing device, and a white line output device. The lane recognition apparatus selects a feature point of a pixel with a large brightness change in an image of a lane surface obtained by imaging means to extract an approximate straight line by Hough conversion, and, thus, to determine this straight line as an edge of a white line. Further, the lane recognition apparatus is operated to search, from a final pixel near the straight line as a starting point, pixels in an image direction far away from a vehicle and determine the pixels as the white line edge.
Meanwhile, in the stud-shaped lane mark, the number of edge features obtainable is extremely small in comparison with the liner lane mark. Thus, the stud-shaped lane mark cannot be recognized well with the above-mentioned method. Thus, as a general method, an estimated position of the stud-shaped lane mark is detected using a filter which returns a value only to features similar to those of the stud-shaped lane mark in an image, and a lane boundary line is recognized based on information of the estimated position.
For example, in the technique disclosed in Patent Document 2, a previously obtained image of the stud-shaped lane mark as a feature extraction filter is used as a template, and a template matching method in which a region having a similar feature is extracted from an image taken by a camera mounted in a vehicle is used. Further, in the technique disclosed in the Patent Document 2, in order to compensate for a deficiency in candidate regions, there has been proposed a method of detecting the stud-shaped lane mark using an image obtained by composing a first image and a second image obtained when a vehicle travels from the position of the first image by a distance of one-half an installation interval of the lane mark.
Patent Document 3 proposes a method of creating a smooth image obtained by composing time-series feature images, making a stud-shaped lane mark linear on the composed image, detecting a feature using an edge gradient and so on as in a linear lane mark, and detecting a straight line using Hough conversion on a feature coordinate, and thereby recognizing a traffic lane position.